Seven million Americans are presently living with disability as a result of traumatic brain injury (TBI) with 500,000 new patients admitted every year. TBI is a devastating disability that leads to sensory and motor dysfunction, learning and memory impairment, and cognitive deficits. At the point of injury, these defects result in focal cell losses, however, damage is not limited to this immediate region. In fact, TBI often results in global changes that include reactive glial cells and synaptic dysfunction. Our studies are designed to examine the role of reactive astrocytes in regulating synaptic function after TBI. We hypothesize that ephrins and Eph receptors function in the homeostasis between the astrocytes and pre-/post-synaptic terminals. Following TBI, we believe ephrins and Eph receptors function to regulate synaptic stability and plasticity. To address this hypothesis, we will employ both in vitro and in vivo approaches using genetically modified mice. Specifically, aim 1 will examine the role of ephrins and Eph receptors in regulating astrocytes synthesis and release of glutamate, D-serine, and signaling intermediates in normal and stretch-injured in vitro conditions. These molecules are an important aspect of astrocyte regulated synaptic homeostasis. Aim 2 will examine whether synaptic formation and function differ when grown on a monolayer of sham or injured astrocytes, as well as examining the role of ephrins/Eph receptor by using genetically mutant astrocytes and/or neurons. Finally, aim 3 will examine how our findings translate to the complex CNS environment by examining synaptic formation/function and astocyte functions using gain and loss-of-function approaches and an in vivo controlled cortical impact injury. In summary, our studies are important in determining whether ephrins and Eph receptors are key regulators of synaptic formation and function in the tri-synaptic interactions between astrocytes, pre- and post-synaptic terminals. RELEVANCE (See instructions); Please refer to proposal page '2'for overall project relevance to public health.